inflected forms does, did, done, doing) can be used as an ordinary lexical verb (main verb): Do your homework! What are you doing? Like other non-auxiliary verbs, do cannot be directly negated with not and cannot participate in inversion so it may itself require do-support, with both auxiliary and lexical instances of do appearing together: They didn't do the laundry on Sunday. (did is the auxiliary, do is the main verb) Why do you do karate? (the first do is the auxiliary, the second is the main verb) How do you do? (a set phrase used as a polite greeting) Meaning contribution In the various cases seen above that require do-support, the auxiliary verb do makes no apparent contribution to the meaning of the sentence so it is sometimes called a dummy auxiliary. Historically, however, in Middle English, auxiliary do apparently had a meaning contribution, serving as a marker of aspect (probably perfective aspect, but in some cases, the meaning may have been imperfective). In Early Modern English, the semantic value was lost, and the usage of forms with do began to approximate that found today. Origins Some form of auxiliary "do" occurs in all West Germanic languages except Afrikaans. It is generally accepted that the past tense of Germanic weak verbs (in English, -ed) was formed from a combination of the infinitive with a past tense form of "do", as exemplified in Gothic. The origins of the construction in English are debated: some scholars argue it was already present in Old English, but not written due to stigmatization. Scholars disagree whether the construction arose from the use of "do" as a lexical verb in its own right, or whether periphrastic "do" arose from a causative meaning of the verb or vice versa. Examples of auxiliary "do" in Old English writing appear to be limited to its use in a causative sense, which is parallel to the earliest uses in other West Germanic languages. Some scholars, such as linguist John McWhorter, argue that the construction arose via the influence of Celtic speakers. Others contend that the construction arose as a form of creolization when native speakers addressed foreigners and children. See also English verbs English clause syntax Intensifier References English grammar Word order Syntax Generative syntax